A group of scientists in Europe has discovered a new kind of magnetic structure in which nearby atomic spins form a spiral. The results confirm a nine-year-old theoretical prediction of so-called “spiral spin liquids” and also reveal the existence of an unexpected vortex state that could potentially be exploited in ultra-high-density magnetic-storage devices.
In ferromagnets such as iron, the magnetic moments, or “spins”, of the material line up with one another over large distances – because this alignment lowers the energy within the material. In what are known as “frustrated” magnets, in contrast, atomic spins are positioned such that multiple different arrangements of spins can place the system in its lowest energy (ground) state. Spins therefore continually reorient themselves as the system flips from one ground state to another.
“Spin liquids” are frequently found in frustrated magnets, and have intermediate order. The material itself is crystalline, meaning that its constituent atoms sit at well-defined points on a lattice. However, the orientation of those atoms’ spins fluctuates continually, just as the position of atoms and molecules within a liquid changes from one moment to the next. But like the correlations that exist between nearby molecules in water, so neighbouring spins in this type of material also fluctuate collectively.
Although physicists have been observing spin liquids for years, spiral spin liquids have remained unconfirmed experimentally until now. Predicted by Leon Balents of the University of California in Santa Barbara and colleagues in 2007, they require nearby spins to be correlated such that the orientation of an atom’s spin axis is offset from that of its nearest neighbour by a certain angle. The size of that angle fluctuates continually in time, as well as in space.
Oksana Zaharko of the Paul Scherrer Institute (PSI) in Switzerland and colleagues set out to observe a spiral spin liquid in the material manganese scandium thiospinel (MnSc2S4), which is made up of antiferromagnetic manganese ions held in a frustrated structure by ions of scandium and sulphur. The researchers’ challenge was to make a sample that was large and pure enough so that it unambiguously generated the signature of a spiral spin liquid. This is a “spiral surface” in the diffraction pattern created when the material is bombarded with neutrons at low temperature.
To do this they used a slow and painstaking process to grow single crystals of MnSc2S4 and then combined multiple crystals to obtain a measurable signal. Having taken over a year to amass some 30 mg in total, they placed their sample in the Diffuse Scattering Neutron Time-of-Flight Spectrometer (DNS) at the Jülich Centre for Neutron Science in Germany. Then, as they cooled the sample down to just a few degrees kelvin, they found what they were looking for: clear evidence for the spiral surface in the sample’s diffraction pattern.
According to Zaharko this is the first “unambiguous” proof of a spiral spin liquid. She points out that a group led by team-member Alois Loidl of the University of Augsburg had found evidence for the spiral in powdered MnSc2S4 using neutron diffraction as far back as 2005, but says that Loidl’s group found “just a good hint” rather than a clear signal.
However, while the latest data confirm the existence of spiral spin liquids, they do not support a second prediction made by Balents and colleagues nearly a decade ago. This is the somewhat counter-intuitive notion of “order by disorder”, in which a spin liquid collapses to a unique ground state selected by thermal fluctuations. Instead, the team found that the MnSc2S4 became ordered as it was cooled below 2.3 K, which, says Zaharko, was caused by atomic spins coupling with their third nearest neighbours, as well as their nearest and second nearest neighbours.
Balents says he is “very pleased” that his group’s prediction of spiral spin liquids has finally been confirmed by what he describes as a “beautiful” experimental study. He is not concerned about the absence of proof for order by disorder, explaining that “many other perturbations can perform the ground-state selection at lower temperatures” and noting that evidence for this mechanism has in any case been found in a number of other frustrated magnets.
He adds that Zaharko and colleagues have found “exciting” evidence that the spins in MnSc2S4 create a vortex shape when exposed to a magnetic field, something that he and his colleagues did not anticipate in their model. Zaharko says that this finding might in future yield higher-density disk drives, given that vortices measure only about 5 nm across, much smaller than features in today’s leading drives. But she cautions that practical devices will require “huge work in materials science” to increase operating temperatures.
The research is reported in Nature Physics.